1. Field of the Invention
This invention relates to the treatment of discrete glass sheets. The present invention particularly relates to a method of coating and tempering discrete sheets of glass.
When glass sheets are cut into discrete individual sheets from a continuous ribbon, the peripheral edge of each cut glass sheet is its weakest portion and subject to breakage when the individual discrete sheets are heated and then cooled during a coating or tempering or heat-strengthening operation. In the past, individual, discrete glass sheets have been shaped, coated and at least partially tempered in a single continuous method that uses the heat required to shape the glass sheet by sagging onto a shaping mold to provide the shaped glass sheet with sufficient heat to cause a film forming composition applied onto the hot surface of the bent glass sheet to form a transparent coating on contact with said hot surface, and then chilling the bent coated glass sheet with sufficient rapidity to impose at least a partial temper thereto. Such a technique involving supporting the individual glass sheets on individual molds of the outline support type is disclosed in U.S. Pat. No. 3,762,902 to William E. Wagner and James S. Golightly, wherein sheet movement is intermittent, not continuous.
The heat needed to cause a glass sheet to sag to its desired shape prior to its coating and chilling steps has been found unsuitable for treating flat glass sheets in the past, particularly those formed on a so-called roller hearth where flat glass sheets are conveyed on spaced rotating rollers and while so conveyed are heated to an elevated temperature sufficient for tempering and then cooled rapidly until at least a partial temper is imparted to the moving flat glass sheets. A typical roller hearth type of apparatus is disclosed in U.S. Pat. No. 3,245,772 to James H. Cypher and Charles R. Davidson, Jr.
Prior to the present invention, attempts to coat flat glass sheets before chilling them to impose at least a partial temper to the flat, coated glass sheets have not been completely successful due to waviness in the glass and/or insufficient temper. The prior art considered it necessary to heat each glass sheet initially to an elevated temperature sufficient to compensate for the cooling resulting from the coating step so that the glass sheet after coating remained at a temperature sufficient for subsequent cooling to impart at least a partial temper in the glass. Such excessive heating caused the glass sheets to develop a wavy appearance termed "roll ripple distortion". If the glass sheets were heated to a temperature below which roll ripple distortion occurred, each sheet was likely to cool to below a temperature suitable for tempering during the application of the coating composition and during the transfer of the coated glass sheets to the cooling station.
Furthermore, when spaced glass sheets are conveyed through successive heating, coating and cooling stations, it is necessary to coordinate an intermittent operation of composition application means for applying the coating composition that reacts on contacting a hot glass surface to form the coating as a reaction product as each glass sheet in turn is conveyed past the composition application means. This coordination comprises stopping the application means when said means faces spaces between successive glass sheets, and resuming operation of said application means whenever a glass sheet is aligned with said application means. Otherwise, spray composition is imparted to the spaces between the spaced glass sheets so as to deposit the coating composition on the conveyor rolls in the coating station of the roller hearth. Some of this coating that is deposited on the rolls transfers to the bottom surface of the glass sheets as the latter pass through the coating station. Usually, it is desired to maintain one of the glass sheet surfaces free of any coating, at least during the time that the upper surface of the glass is being coated. Therefore, the prior art technique required an additional step of removing the coating transferred to the glass sheet surface contacting the rotating conveyor rolls exposed to the composition when spaced glass sheets were coated.
It is also known to coat a newly formed continuous ribbon of float glass shortly after the latter leaves a ribbon forming chamber and before it enters an annealing chamber. A typical apparatus for performing such a method is disclosed in U.S. Pat. No. 3,652,246 to Joseph E. Michelotti and Vern A Henery. However, such an apparatus requires that the coated, continuous glass ribbon be annealed so as to enable the ribbon to be cut into discrete sheets after coating. Hence, such an apparatus would require a reheating and rechilling step in order to impart a temper to the coated glass.
In the past, it has been considered necessary to enclose the coating apparatus physically within closed solid barriers forming an enclosure around the coating station to protect the atmosphere in the vicinity of the coating station from contamination by any excess coating composition and reaction product not used to form the coating. Therefore, it was necessary in the past to dismantle at least part of the enclosure whenever it was necessary to service the composition application means, particularly the spray gun forming an essential part of a specific embodiment thereof. In order to avoid the need for such dismantling, the prior art has incorporated at a coating station exhaust ducts that direct excess spray originally imparted vertically downward from a spray gun in a vertically upward direction. Such an arrangement is relatively inefficient and can be improved.
It has also been conventional in the past to use gas curtains extending in vertical planes transverse to the conveyor path to define the upstream and downstream boundaries of the coating station. In the past, these gas curtains have used air at ambient temperature conditions. The combination of relatively cool air and relatively cool coating composition applied to the glass as in the prior art does not impair the coating unduly or cool the glass to a temperature unsuitable for commencing an annealing operation of the coated glass. However, the exposure of the moving glass sheets to two curtains of cool air and a cool coating composition may cause the glass sheets so treated to cool below a temperature suitable for imparting an adequate temper by subsequent rapid cooling.
It has been traditional in the flat glass industry to form a continuous ribbon of uncoated glass and cut the glass to individual discrete sheets having desired outlines of predetermined or preselected size before tempering the latter. The use of tongs to suspend glass sheets during thermal treatment involved in tempering has caused marking and distortion of the flat glass at the tong gripping points. Therefore, the flat glass industry has used roller hearths to convey individual glass sheets during thermal treatments needed to temper or to coat and heat strengthen said glass sheets. However, prior to the present invention, the flat glass industry still required a continuous roller hearth line that treated individual discrete flat glass sheets by successive heating, coating and tempering steps that was suitable for mass production that did not result in considerable glass sheet breakage and/or roll ripple distortion and/or inadequate tempering and/or the need to remove undesired coating from the undersurface of the glass in contact with the conveyor that transported the glass through the coating station, that provided more ready access to the spray gun and more efficient removal of spray at the coating station.
2. Description of the Prior Art
U.S. Pat. No. 3,652,246 to Joseph E. Michelotti and Vern A. Henery; U.S. Pat. No. 3,660,061 to Harold E. Donley, Raymond G. Rieser, and William E. Wagner; U.S. Pat. No. 3,679,386 to Akira Kushihashi and Koji Ikeda; U.S. Pat. No. 3,689,304 to Charles R. Bamford; and U.S. Pat. Nos. 3,841,858 and 3,887,349 to Naotomo Akashi and Kutsuji Fujimoto all disclose apparatus for coating a newly formed continuous ribbon of glass. The ribbon is annealed after it is coated so that it can be cut after it is coated. In order to temper annealed coated glass sheets, it has been necessary to reprocess the glass on another apparatus to reheat each glass sheet to tempering temperature followed by rapid cooling.
In the patents enumerated in the preceding paragraph, a spray gun applies a coating composition in a downward direction against the upper surface of a moving continuous glass ribbon and the spray gun is flanked by evacuation means that remove excess spray in an upward direction from the upper surface of the ribbon. The efficiency of such an arrangement leaves something to be desired.
In the aforesaid U.S. Pat. Nos. 3,689,304 and 3,887,349, air barriers are provided at the upstream and downstream boundaries of the coating station. It is assumed that the air supplied for such barriers is drawn from the surrounding atmosphere and therefore may shock the glass before the final cooling step of tempering, thus tending to cause breakage.
U.S. Pat. No. 3,185,586 to Arnold E. Saunders and William E. Wagner and U.S. Pat. No. 3,410,710 to John M. Mochel disclose compositions for coating discrete glass sheets that may be flat. Such compositions are typical of those that are applied in the practice of this invention and the disclosures of these patents are incorporated by reference to provide a disclosure of suitable coating compositions for performing the present invention.
U.S. Pat. No. 3,806,331 to Albert Bezombes discloses a roller hearth suitable for heating and chilling glass sheets that are discrete to impart a temper to individual flat glass sheets. This patent does not involve coating. U.S. Pat. No. 3,672,861 to George S. Ritter, Jr., and Frank J. Hymore is another patent disclosing a typical roller hearth operation for heating and cooling flat glass sheets in order to impart a temper without coating.
Various patents to conveyors have been granted in which glass sheets are conveyed at different speeds along different portions of the conveyor. These include U.S. Pat. No. 1,482,106 to Clarence W. Avery and Wenzel G. Vasey; U.S. Pat. No. 1,638,769 to Halbert K. Hitchcock; U.S. Pat. No. 1,848,102 to James C. Blair; U.S. Pat. No. 2,025,102 to Alfred L. Harrington; U.S. Pat. No. 3,452,865 to Anthony S. Brittain; U.S. Pat. No. 3,594,149 to Alan Pickavance and Ronald Charles Freestone and U.S. Pat. No. 3,792,993 to Arvi Artama and Erkki Artama. Other patents to conveyors which control the spacing between the work pieces other than glass sheets to be conveyed include U.S. Pat No. 1,457,352 to Charles H. Breher; U.S. Pat. No. 2,488,104 to Allan A. Tunley; U.S. Pat. No. 3,072,095 to Harold E. Keessen and William B. Morgan, Jr.; and U.S. Pat. No. 3,332,819 to Eugen Siempelkamp.
Despite the existence of so many patents relating to multiple speed conveyors for glass sheets and other articles, there was no patent available in the prior art to teach one skilled in the flat glass coating and tempering art how to coat and temper a series of flat glass sheets while moving the latter continuously on a roller hearth conveyor without obtaining an excessive amount of breakage or requiring the removal of a coating from the opposite surface from the surface desired to be coated, while avoiding roll ripple distortion and insufficient tempering and minimizing the amount of excess spray that escapes to the environment adjacent the coating station even though the latter is exposed so as to enable ready access to coating application equipment, such as spray guns, that require frequent maintenance.
In addition, the prior art patents lacked a teaching of glass sheet treatment in which continuously moving, discrete glass sheets are spaced apart while subjected to heating and cooling and brought into abutting relation for a coating step.